Useful Utilities?

kberkit asks: "I've just started as a Web designer/Swiss-army-knife, In the crash course of my travails, I've come across some nifty online tools: traceroute to find out what route and how long my Web pages are taking to load to outside servers; DNS lookup, to figure out what IP address a page is coming from; and online HTML validators to make sure my code is clean. I'm sure this is old hat to a lot of Slashdotters, but I'd be interested in knowing what other online tools others would suggest for us poor schleps who find ourselves expected to work miracles with little budget for proper software." For those of you who like traceroute and ping, may I recommend mtr as a supplement for your toolbox? What other small utilities have you all found useful in your day-to-day work lives?